Microwave signals within a partitioned module generally radiate to fill each cavity with some level of microwave energy. The level within an individual cavity is determined by the cavity dimensions, materials, and structures, and to a large extent by the means by which the microwave signal excites the cavity. Careful design methods will minimize but not eliminate these effects. Cavity isolation denotes the extent to which microwave energy arising from one cavity can be minimized in another cavity within the module.
When several separate cavities are required in a partitioned module, and very high levels of cavity-to-cavity isolation are required, special techniques must be implemented since the microwave energy present in one cavity will "leak" over into an adjacent cavity through several mechanisms. Skin effect leakage is easily controlled by proper wall thickness selection. Leakage through via holes can generally be tolerated, but can be minimized by shielding and filters. Leakage through cracks between the lid and the housing comprising the module can be a serious problem, however. Any crack will let energy through in a manner similar to light leaking through cracks in a door jam in a darkened room. Although such leakage is not normally a problem, when high adjacent cavity isolations are required to prevent unwanted noise or oscillation, the solution has typically been to weld, solder, or epoxy the lid in place, thus eliminating the crack through which the energy can leak. Gaskets and spring-loaded fingers have also been used. Each of these methods, however, has serious limitations. Such limitations include difficulty in assembly and removal for rework, as well as utilization of expensive capital equipment and various cleaning methods.
Thus, what is needed is a method and apparatus for high microwave isolation without the need for increased processing and difficult or cumbersome rework procedures. It is also desirable that such a method and apparatus provide high isolation with a simple lid that may be screwed on. It is also desirable that such a method and apparatus allow the housing to be tuned, tweaked, modified, and/or reworked without processing to altering the original housing configuration. An additional desirable feature would be to allow such a housing to be replaced with a metal coated plastic housing, where isolation is very difficult to obtain. The lids in such a case would not have to be conductively attached, but could use any good non-conductive adhesive appropriate for mechanically bonding the lid to the housing.